A contract is only worth as much as you are willing to pay to enforce it. It makes sense to pay an army of lawyers for years to pursue a $100m breakup fee. For $100k, not so much.
The solution though is escrow. Upon agreement, you have them wire the money to an escrow agent. If they back out, the escrow goes to you. There's no need for follow-on lawyering. (And no, no escrow agent worthwhile is going to be pressured easily into giving a large corporation their $100k back).
Quod licet Iovi, non licet bovi